The active implantable devices associated with the present invention typically include a housing, generally designated as a “generator”, that is electrically and mechanically connected to one or more leads. The leads are equipped with electrodes that are intended to come into contact with the patient's myocardium at those sites where the electrical potentials are detected (collected) and/or the stimulation pulses are delivered (applied). These electrodes can be endocardial electrodes (e.g., electrodes that are placed in a cavity of the myocardium in contact with the wall of the myocardium), epicardial electrodes (e.g., electrodes that are preferably used to define a reference potential, or to apply a shock stimulation pulse), or intravascular electrodes (e.g., electrodes that are introduced into the coronary sinus and advanced to a position that faces the myocardial wall of the left ventricle).
Heretofore, an MRI examination was contraindicated for patients having an implanted cardiac pacemaker or defibrillator. This is for several reasons, including, for example:                heating near the electrodes connecting the generator to the patient's heart;        forces and torques of attraction exerted on the device immersed in high intensity magnetic fields generated by an MRI equipment; and        unpredictable behavior of the device itself, due to exposure to extreme magnetic fields.        
Regarding the unpredictable behaviour of a device when exposed to magnetic fields, there have been a number of solutions that resolve this problem. It is indeed desirable that when exposed to static and alternating electromagnetic fields that are generated by a conventional MRI examination, the device behavior is predictable and known in advance. The following problems are likely to occur in such a situation:                an erratic detection of a static electromagnetic field generated during an MRI examination, especially for a device equipped with a Reed magnetic switch. A Reed magnetic switch detects the presence of a permanent magnet in proximity to the device, normally used by a practitioner after implantation to put the device in a safe operating mode, for example, when using an electric scalpel, or to evaluate the state of the battery charge depletion of a device (in a magnet mode, the stimulation frequency is fixed and reflects the level of battery charge). A Reed magnetic switch detects a static electromagnetic field of a relatively low amplitude, but has a totally unpredictable behaviour in an MRI environment where the magnetic field is typically stronger by a thousand times than that of a permanent magnet;        the deterioration of its intrinsic performance;        the dynamic signals emitted by the MRI instrument can be detected by the device and misinterpreted as cardiac signals.        
In this latter regard, it is necessary to take into account the fact that throughout the duration of an MRI examination—which can last several minutes—the device shall nevertheless remain functional and provide if necessary seamless stimulation of the myocardium.
It is therefore necessary to have means of detection and of management of such a situation, providing the following functions:                indicating to the device that the patient will be subjected to MRI magnetic fields;        inhibiting the circuits of the device that may be disturbed by the electromagnetic fields emitted by the MRI instrument; and        operating the device in a dedicated pacing mode, tailored to the patient and compatible with the electromagnetic fields produced by the MRI instrument.        
The present invention relates more particularly to the solution to the first function. The second and third solutions may be implemented independently or in combination with the first function.
A first known approach involves an external programmer to indicate to the device that it will be exposed to electromagnetic fields of an MRI instrument and should therefore adopt a particular configuration, both for its own operation and for the delivered stimulation. The difficulty of this approach is that it requires a programmer prior to the test, and thus it requires interruption by a qualified practitioner. In addition, the device needs to be reprogrammed to its original pacing mode after the MRI examination. Otherwise, the patient could leave the MRI center with a device operating in a mode that is not configured for everyday life. Thus, this technique suffers from the pervasive risk that the MRI center fails to reprogram the device before the patient leaves.
Another known approach in the art is to equip the device with means for automatically detecting a magnetic field. As described above, implantable cardiac devices are typically equipped with a detector of a weak magnetic field (also referred to herein as a “weak field”), sensitive to the presence of a magnet (e.g., a permanent magnet) placed near the device to place it in a “magnet mode”. Commonly used detectors of weak field use a Reed magnetic switch. A Reed magnetic switch is too large and unpredictable in the presence of a strong static field of the MRI type. Other types of commonly used detectors of weak fields are integrated sensors such as MAGFET type components described e.g. in WO 94/12238 A1 (which have the disadvantage of consuming energy to operate), or micro electromechanical sensor (MEMS) devices, as is described, e.g., in FR 2 805 999 A1.
These various sensors used to detect a weak field (typically about 1.5 mT) are unsuitable for detecting strong fields such as those produced by MRI instruments (typically between 0.5 T and 3 T or more) which are nearly a thousand times stronger, and are located in areas where conventional sensors present a non-linear response. Indeed, a weak field sensor may be “deaf” to the presence of a strong field.
Special techniques have been proposed to detect the strength of a static magnetic field of an MRI type. The U.S. Published Application 2007/191914 A1 describes a device detecting the presence of a strong static magnetic field by analyzing the impedance of an inductive component, such as a coil of an inductive switching regulator: the presence of a strong magnetic field has the effect of saturating the core of the inductive component, causing a change in impedance that is detected by the device.
WO 2006/124481 A2 describes a technique of detecting the presence of an MRI field by measurement of the voltage collected by the terminals of a telemetry antenna as well as on a lead.
These devices, however, cannot overcome the difficulties outlined above, especially with the high degree of reliability required for an implanted device intended to fully and automatically operate in a strong electromagnetic field.
EP 1 935 450 A1 describes another technique of using giant magnetic resistive type (GMR) devices associated in a Wheatstone bridge. The Wheatstone bridge plays the role of a single strong/weak field mixed sensor: indeed, the balance of the bridge is more or less altered by the strength of an electromagnetic field, and changes of the electromagnetic resulting from the differential voltage is analyzed by a converter placed at the output of the Wheatstone bridge to give an overall estimate of the field level.